Observation of phosphorescence from tris(8-hydroxyquinoline) aluminum thin films using triplet energy transfer from iridium complexes

Abstract

Strong phosphorescence from tris(8-hydroxyquinoline) aluminum $(\mathrm{Al}{\mathrm{q}}_{3})$ was observed in amorphous $\mathrm{Al}{\mathrm{q}}_{3}$ thin films doped with phosphorescent molecules, fac tris(2-phenylpyridine) iridium $[\mathrm{Ir}{(\mathrm{ppy})}_{3}]$. The energy-transfer and light-emission mechanisms were studied in the $\mathrm{Ir}{(\mathrm{ppy})}_{3}$-doped $\mathrm{Al}{\mathrm{q}}_{3}$ thin films. The triplet energy of $\mathrm{Al}{\mathrm{q}}_{3}$ was determined to be $2.03\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ from the highest-energy peak of the phosphorescence spectra, and the phosphorescence lifetime was $5.3\phantom{\rule{0.3em}{0ex}}\mathrm{ms}$ at $8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The phosphorescence lifetime and intensity significantly decreased with an increase in temperature, especially at temperatures above $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. From this study, we have found that the triplet energy transfer from $\mathrm{Ir}{(\mathrm{ppy})}_{3}$ enables us to characterize the phosphorescence properties of $\mathrm{Al}{\mathrm{q}}_{3}$, and that $\mathrm{Ir}{(\mathrm{ppy})}_{3}$ plays the important role of a phosphorescent sensitizer for $\mathrm{Al}{\mathrm{q}}_{3}$.